The present invention relates to a pre-tensioned spring for loading a track tensioning roller for a ground drive track on a track laying vehicle or track drive system that provides two different levels of tensioning loads. A lower track tension is provided for normal operation, and a higher track tension is provided to prevent detracking and buckling of the track during high tractive effort conditions or when debris is forced between the track and the drive sprocket, idlers or track support rollers.
Ground support and drive tracks are used in various vehicles, for good traction and flotation. The tracks are endless members driven by a drive wheel or sprocket, and supported on a plurality of rollers on a lower length of the track. The track has to be kept under tension, and heavy springs or hydraulic or grease filled cylinders have been used for such purposes.
Some track tensioners utilize two separate springs to attempt to provide for two-level tensioning. For example, springs provided in series, one of which is lesser force than the other, are utilized as a shock absorber in Japanese abstract No. JP 55156773. The assignee of the present application owns U.S. Pat. No. 6,322,171 which shows a spring loaded track tensioning assembly. However, a reliable two stage spring system is desired for providing a lower average track tension for normal use, and higher tension only when it is needed.